usepackage{tikz,tikz-qtree-compat}
    tikzset{baseline=0pt,level distance=20pt,sibling distance=0pt}
begin{document}
    Tree [.CP [.who ] [ [.$lambda_1$ ]
        [.IP [.$t_1$ ] [.registered ] ] ] ]
end{document}

usepackage[linguistics]{forest}
newcommand{tree}[1]{
    begin{forest}
        for tree={
        inner sep=0pt,
        % where n children=0{font=itshape}{},
     %    calign=fixed edge angles,
        parent anchor=south,
      },
      before typesetting nodes={% page 52: example (81)
        where content={}{% shape=coordinate gives an error if used here but this is *almost* right - it just leaves a little, tiny gap
          text width=.001pt,
          inner sep=0pt,
          before drawing tree={% here we make sure that the tiny gap disappears so only the size is not quite dimensionless
            shape=coordinate,
            typeset node,
          },
          for parent={
            for children={
              anchor=north,
            }
          }
        }{}
      }
      #1
    end{forest}
}

begin{document}
    tree{
        [CP [who] [ [$lambda_1$]
        [IP [$t_1$] [registered]]]]
    }
end{document}